A number of molybdenum-based catalysts for oxidation reactions of organic compounds are known. Examples of such catalysts include P.Mo.Bi catalysts as disclosed in JP-B-36-3563 (corresponding to U.S. Pat. No. 2,941,007) and JP-B-36-5870 (corresponding to U.S. Pat. No. 2,904,580) (the term "JP-B" as used herein means an "examined published Japanese patent application"); P.Mo.Fe.Bi catalysts as disclosed in JP-B-38-17967 (corresponding to U.S. Pat. No. 3,226,422) and JP-B-39-3670 (corresponding to U.S. Pat. No. 3,171,859); Mo.Te catalysts as disclosed in JP-B-37-11008; Mo.Bi.Sb catalysts as disclosed in JP-B-39-10111; Mo.Bi.Pb catalysts as disclosed in JP-B-42-7774; and Mo.Bi.Cr catalysts as disclosed in JP-A-50-64191 (corresponding to U.S. Pat. No. 4,174,354) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Further, several improvements have been proposed with respect to catalyst activity as disclosed, e.g., in JP-B-47-27490 (corresponding to U.S. Pat. No. 3,959,384), JP-B-54-22795 (corresponding to U.S. Pat. No. 3,984,477), and JP-B-60-36812 (corresponding to U.S. Pat. Nos. 4,600,541 and 4,377,534).
Many of these catalysts have disadvantages such as low catalytic activity and strength when applied to a fluidized bed reaction. Hence, various proposals have been made on a process for producing the catalyst. For example, JP-B-37-8568 (corresponding to U.S. Pat. No. 3,044,965) discloses a method of adding at least 1% by weight of ammonium nitrate to a slurry containing catalyst components; JP-B-57-49253 (corresponding to U.S. Pat. No. 3,746,657) discloses a method of adding a silica sol stepwise to a molybdenum aqueous solution at a temperature of not higher than 66.degree. C.; JP-A-56-44046 (corresponding to U.S. Pat. Nos. 4,453,006 and 4,280,929) discloses a method of using fumed silica; JP-B-54-12913 discloses a method of preparing a molybdenum-bismuth-iron-sodium-phosphorus catalyst within a limited narrow rang of composition; JP-A-57-65329 (U.S. Pat. No. 3,872,148) discloses a method for preparing an attritionresistant molybdenum-bismuth-antimony fluid-bed catalyst which comprises spray drying a catalyst component-containing slurry having a pH of 1 or less and a temperature of 60.degree. C. or lower; JP-A-53-10388 discloses a method comprising mixing a molybdic acid aqueous solution with a solid bismuth compound and adjusting the pH of the slurry to 8 to 10; and JP-B-59-51848 (corresponding to U.S. Pat. No. 4,418,007) discloses a method comprising simultaneously adding a bismuth salt aqueous solution and aqueous ammonia to a molybdic acid aqueous solution having a pH of from 6 to 8.
However, the so far proposed processes for producing a catalyst are not always satisfactory in achieving both catalytic activity and strength. In particular, the conventional processes for producing a molybdenum-containing catalyst give rise to various problems when applied to the production of molybdenum-based catalysts as desired in the present invention. For example, the catalyst prepared according to the process of JP-B-54-22795 (corresponding to U.S. Pat. No. 3,984,477) failed to achieve satisfactory reaction results as showing only a low acrylonitrile yield and a higher ammonia combustion as proved in Comparative Examples 5 and 6 hereinafter described. Further, where a mixture of catalyst raw materials is merely adjusted to a pH between 8 and 10, gelation of a silica sol proceeded to increase the viscosity of the slurry so that the slurry cannot be sufficiently spraydried as demonstrated in Comparative Examples 1 to 3 hereinafter described. In this case, also, satisfactory results cannot be obtained with respect to physical properties of the catalyst, particularly strength.